The present invention relates to communication systems using non-geosynchronous satellites and, more particularly, to a system which determines and registers the location of a mobile terminal.
In a mobile satellite communication system using satellites, a mobile terminal communicates via the satellites and land earth stations. FIG. 9 conceptually shows a mobile terminal, a land earth station and a satellite. In FIG. 9, reference numeral 1 denotes a mobile terminal, 20 a land earth station, 30 a communications satellite (hereinafter referred to simply as a satellite) capable of communicating with the mobile terminal 1 and the land earth station 20, 30-1 through 30-7 spot beam coverage areas wherein the satellite 30 is capable of communicating with mobile terminals, 6 is a terrestrial network which has at least one land earth station, 7 is a switching facility, 8 a public switched telecommunication network, and 9 is a subscriber telephone set. In a mobile telecommunication system using satellites, a mobile terminal is allowed to originate a call as long as it is at a place where at least one satellite used in the satellite communication system is seen. In a case where a call for a mobile terminal is originated, however, it is necessary to send a call announcement message to the desired mobile terminal to connect the call. The International Maritime Satellite Organization standard A system, which is a mobile satellite communication system now in service, employs a total of four geosynchronous satellites covering the Indian Ocean Region, the West Atlantic Ocean Region, the East Atlantic Ocean Region and the Pacific Ocean Region, respectively. Each satellite irradiates a global beam to cover its responsible Region to communicate with mobile terminals. To send a call announcement message to the called mobile terminal, the INMARSAT standard A system uses a method which specifies or designates the Ocean Region where the mobile terminal is assumed to be located, by the corresponding number in front of the terminal number.
With such a method, however, if the desired mobile terminal is not located in the specified Ocean Region, the calling party will have to dial again specifying another Ocean Region or initiating a call. In the case of employing this method in a telecommunication system with a large number of mobile terminals, the smaller the number of regions that the calling party specifies by dialing, the larger the number of circuits that must be prepared for sending call announcement messages in each area. Moreover, the larger the number of regions, the more times of dialing may be required for the calling party reach the desired mobile terminal. It is desired, therefore, to implement a method in which the calling party only has to dial the terminal number and the right region is automatically selected. This requires location information of all the mobile terminals in use. In a mobile communication system using geostationary satellites having a plurality of spot beams, the area wherein a mobile terminal is allowed to communicate with the satellite does not move with respect to the earth, and hence the spot beam coverage area wherein the mobile terminal is located can be used intact as location information. In a communication system using non-geosynchronous satellites, however, the area wherein a mobile terminal is capable of communicating with the satellite changes every moment, and hence the spot beam coverage area cannot be used as location information.